The present invention relates generally to a method and apparatus for evaluating magnetic cores, and, more specifically to a method and apparatus for detecting magnetic anomalies in the magnetic cores used for the differential transformers employed in ground fault circuit interrupters.
A differential transformer has an opening in the middle through which the wires being monitored thread through. These wires are arranged so that the outgoing current through one wire produces a magnetic field in the differential transformer which is cancelled by the magnetic field set up by the incoming current through the other wire. If both currents are equal, the magnetic flux in the differential transformer is zero. If the two currents are not exactly equal, a net magnetic flux is established in the differential transformer which produces a voltage in a sense winding wound around the differential transformer itself, and which voltage then constitutes a signal to trip a ground fault circuit interrupter at a pre-established value of difference current.
It is well known that the magnetic cores used for the differential transformers employed in ground fault circuit interrupters, such as those marketed by the assignee of the present invention, must be homogeneous and of a high permeability. In particular, a departure from homogeneity results in the differential transformer being sensitive to magnetic fields other than those caused by current in the wires passing through the center of the toroidal core used in the transformer, thus rendering the finished ground fault circuit interrupter unacceptable. These stray magnetic fields may be produced by the current in the wires threading the center of the toroidal core from points on the wire distant from the center of the core, and maybe at an angle to the core caused by bending of the wires themselves. Ordinarily, this type of defect is not detected until the finished product is assembled and tested.
"Load Shift" is defined as a change in trip current in a ground fault circuit interrupter from no load to full load conditions. Depending upon whether the trip current (fault) is introduced between line phase to load neutral, or load phase to line neutral, the phenomenon will add to, or subtract from, in a generally symmetrical manner, the trip current measured at no load. In the first case, the neutral current will be larger than the phase current at the point where the two wires thread the differential transformer, while in the second case the phase current will be larger than the neutral current at that point.
Thus, a product which should trip between 4 and 6 milliamperes may measure between that specification at no load and be out of that specification at some value of load current, with the most common measurement at the maximum rating of 20 amperes. In some cases, the device will trip with no fault current applied during a 120 amp surge current test, meant to simulate motor inrush current.
It is thus clear that what is needed is a reliable and accurate way of evaluating the component cores prior to the labor intensive and expensive manufacturing steps involved in the wiring and assembly of the cores into the finished ground fault circuit interrupter product. Prior to the present invention it is believed that said need has gone unfulfilled.
Accordingly, it is a general object of the present invention to provide a method and apparatus for evaluating magnetic cores.
It is a specific object of the present invention to overcome the aforementioned difficulties and to fulfill the long felt needs by providing a method and apparatus for detecting magnetic anomalies in magnetic cores of the type used for the differential transformers employed in ground fault circuit interrupters.
It is a further object of the invention to provide a method and apparatus for evaluating the permeability and homogeneity of magnetic cores of the type used for the differential transformers employed in ground fault circuit interrupters.
Other objects of the invention will be apparent in the following detailed description and practice of the invention.